The invention relates to a method and a device for removing the ambiguity over range and speed, used more especially in a Doppler radar transmitting pulses at several repetition frequencies.
Present Doppler radars transmit pulses with repetition frequencies relatively close one to another. The use of several repetition frequencies is justified by the fact that a single repetition frequency introduces ambiguity over the range and speed. On the one hand the repetition period determines the maximum range D' without ambiguity, D'=CT/2, where C is the speed of light, and, on the other, the true Doppler frequency F.sub.D is measured with an error equal to a multiple of the repetition frequency, F.sub.D =f.sub.D +k.multidot.f.sub.R, where f.sub.D is the Doppler frequency, f.sub.R =1/T the repetition frequency being considered and k a whole number.
In general, the range ambiguity is eliminated by the use of a repetition frequency sufficiently low for the range D' to be at least equal to the maximum range D.sub.o of the radar.
To remove the ambiguity over the Doppler frequency, several repetition frequencies are used sufficiently close one to another for the ambiguity numbers of the same target to differ by 1 at the most; for all other values the ambiguity remains.
Thus through this restriction at close repetition frequencies one cannot use the advantages due to repetition frequencies distant one from another as, for example, the rejection of ground echoes due to the antenna main lobe.